1. Field of the Invention
The present invention generally relates to the field of thermocline storage systems and more particularly to improvements in the structure and operation of a storage tank for such systems.
2. Description of the Prior Art
A common design for a simple thermocline storage system includes a single thermal storage tank, lines connecting it with a source of thermal heat (e.g., solar collectors), and a discharge heat exchanger. The lines typically form a closed system; and in operation, thermal energy (i.e., heat) is supplied from the solar or other sources to heat a liquid being circulated in the system. The liquid is the storage medium and once heated, it is stored in the top of the storage tank above a volume of the same liquid at a colder temperature.
Since the hotter liquid is less dense than the colder, the hotter and colder volumes of liquid stratify in the storage tank with the hotter above the colder. In doing so, a natural interface region or thermocline is formed between the two volumes of liquid. This natural interface region extends substantially horizontally and moves vertically relative to the tank during the operation of the system. For example, during the charge cycle, cold liquid is withdrawn from the bottom of the tank, heated in a heat exchange with the heat source (e.g., solar collectors), and returned to the top of the tank. During charge, the interface region moves down until it reaches the bottom of the tank at which time the tank is then fully charged. During discharge, hot liquid is withdrawn from the top of the tank, cooled in the discharge heat exchanger (transferring heat to the end use load), and returned to the bottom of the tank. During this time, the interface region moves up until it reaches the top of the tank at which time the device is then fully discharged.
Thermal storage designs similar to the simple thermocline described above include a membrane or bladder tank, mixed tank, and a two design. In the membrane or bladder tank, a flexible membrane is fixed about its perimeter to the interior of the tank roughly at the middle height of the tank. As the tank is charged, the membrane distends downwardly to form a downward-extending, concave shape, and conversely, the membrane distends upwardly into an upward-extending, concave shape during discharge. The membrane creates an artificial interface region during operation which does not extend horizontally but rather follows the shape of the membrane (i.e., concave downwardly during charge and concave upwardly during discharge). Membrane tanks permit no passage of liquid by the membrane and typically have an Ullage factor (representing the unusable percentage of the tank volume) on the order of 20%.
In a mixed tank design, the hot and cold liquids are continuously mixed during both charge and discharge. The whole tank is then at the same temperature at any point in time. As compared to a simple thermocline unit in a comparable system, the mixed tank design has several drawbacks. First, the mixed tank design produces a lower maximum temperature in the stored liquid available for end use. Second, because cold liquid enters and is mixed with the stored liquid in the tank during discharge, the temperature of the discharging liquid continuously drops wherein only low temperature heat is generally delivered for most of the discharge cycle. Third, during charging, the cool liquid leaving the tank is at a higher temperature than in a comparable thermocline design and, therefore, larger lines and pumping power as well as heat exchangers typically must be used in order to absorb the heat produced by identical solar collectors, particularly at peak periods. Due to the mixing and the changing output temperature levels in a mixed tank design, the average temperature from the heat source must be higher to match the performance of a thermocline unit. From a pure collection standpoint, for example, and in a like system using flat plate solar collectors, the net energy collected with a thermocline unit can be on the order of 20% greater than with a mixed tank design.
The two tank approach has basically the same performance advantages as the thermocline design; however, the fundamental drawback of the two tank approach is that it requires an additional tank. Further, with the two tank design, both tanks must be sized to hold the entire volume of the liquid for fully charged and fully discharged conditions. The quantity of storage liquid is approximately the same but with the thermocline design, the cost for tanks is cut in half. Further, the thermocline design has less heat loss due to the elimination of the second tank. The primary reduction in heat loss is due to the elimination of the surface area of the second tank. It also typically has a lower average operating temperature in that both systems are commonly held in the discharged state longer than they are held in the charged state. Consequently, with minimum and maximum temperatures of, for example, 100.degree. F. and 150.degree. F., the average operating temperature of the two tank approach is roughly 125.degree. F. whereas the thermocline design is more on the order of 120.degree. F. or less. For a total system over time, the smaller surface area and lower average operating temperature of the thermocline approach offer input power savings while offering substantially identical output performance.